Assignment of wireless coverage areas based on media codec

ABSTRACT

A radio access network (RAN) may receive a call setup request from a wireless communication device (WCD). The call setup request may indicate that the WCD supports a first media codec. The RAN may obtain a set of candidate wireless coverage areas for serving the WCD. A first subset of the candidate wireless coverage areas may support the first media codec, and a second subset of the candidate wireless coverage areas might not support the first media codec. The RAN may assign traffic channels to the WCD, such that the assigned traffic channels include traffic channels from at least two of the first subset of the candidate wireless coverage areas, but do not include traffic channels from any of the second subset of the candidate wireless coverage areas. The RAN may communicate with the WCD substantially simultaneously via the assigned traffic channels using the first media codec.

BACKGROUND

Wireless service providers typically design their wireless networks tocomprise a number of partially-overlapping wireless coverage areas. Eachwireless coverage area may support one or more types of media codecs(e.g., voice, music, still image, and/or video codecs). As new,higher-quality and/or more efficient media codecs are deployed, wirelessservice providers may upgrade their base transceiver stations (BTSs)and/or its base station controllers (BSCs) to support these new mediacodecs.

OVERVIEW

During a call, a wireless communication device (WCD) may communicatesubstantially simultaneously via a number of wireless coverage areas.For instance, the WCD may receive the same information in two or morediscrete messages from two or more different wireless coverage areas atapproximately the same time. This substantially simultaneouscommunication may improve the reliability of communication involving theWCD.

However, in some cases, BTSs and/or BSCs that define these wirelesscoverage areas may be incrementally upgraded, resulting in some wirelesscoverage areas supporting the new media codecs, and other wirelesscoverage areas not supporting these media codecs. Thus, a WCD seeking touse a new media codec may not be able to do so via all availablewireless coverage areas. In scenarios in which some available wirelesscoverage areas support the new codec and others do not, the WCD mayresort to using an old media codec, instead of the new media codec. Inthis way, the WCD can engage in substantially simultaneous communicationvia any combination of the available wireless coverage areas.

Such an arrangement may limit the use of new media codecs, which, inturn, may reduce the efficiency of wireless network operation.Therefore, it may benefit both wireless network operators andsubscribers for a radio access network (RAN) to attempt to assignwireless coverage areas that support one or more new media codecs toWCDs that support the same codecs.

Accordingly, in a first example embodiment, a RAN may receive a callsetup request from a WCD. The call setup request may indicate that theWCD supports a first media codec. The RAN may obtain a set of candidatewireless coverage areas for serving the WCD. A first subset of thecandidate wireless coverage areas may support the first media codec, anda second subset of the candidate wireless coverage areas might notsupport the first media codec. Possibly based on (i) the WCD and thefirst subset of the candidate wireless coverage areas supporting thefirst media codec, and (ii) the second subset of the candidate wirelesscoverage areas not supporting the first media codec, the RAN may assigntraffic channels to the WCD. The assigned traffic channels may includetraffic channels from at least two of the first subset of the candidatewireless coverage areas, and no traffic channels from any of the secondsubset of the candidate wireless coverage areas. The RAN may communicatewith the WCD substantially simultaneously via the assigned trafficchannels. The communication between the RAN and the WCD may use thefirst media codec.

In a second example embodiment, a RAN may receive a handoff request froma WCD. The handoff request may indicate that the WCD supports a firstmedia codec. The WCD may be served by first set of wireless coverageareas, each of which supports the first media codec. A second set ofwireless coverage areas that are candidates for adding to the first setmay be obtained. A particular candidate wireless coverage area maysupport the first media codec, and other candidate wireless coveragearea(s) might not support the first media codec. Possibly based on (i)the WCD and the particular candidate wireless coverage area supportingthe first media codec, and (ii) the other candidate wireless coveragearea(s) not supporting the first media codec, the RAN may add theparticular candidate wireless coverage area to the first set, but notadd the other candidate wireless coverage area(s) to the first set.After adding the particular candidate wireless coverage area to thefirst set, the RAN may communicate with the WCD substantiallysimultaneously via the first set of wireless coverage areas. Thecommunication between the RAN and the WCD may use the first media codec.

These and other aspects and advantages will become apparent to those ofordinary skill in the art by reading the following detailed description,with reference where appropriate to the accompanying drawings. Further,it should be understood that this overview and other descriptionthroughout this document is merely for purposes of example and is notintended to limit the scope of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a RAN, in accordance with an example embodiment;

FIG. 2 depicts a RAN radiating to define several wireless coverageareas, in accordance with an example embodiment;

FIG. 3 is a block diagram of a RAN component, in accordance with anexample embodiment;

FIG. 4 is a first message flow diagram, in accordance with an exampleembodiment;

FIG. 5 is a second message flow diagram, in accordance with an exampleembodiment;

FIG. 6 is a first flow chart, in accordance with an example embodiment;and

FIG. 7 is a second flow chart, in accordance with an example embodiment.

DESCRIPTION I. Network Architecture

FIG. 1 shows a simplified block diagram of a wireless communicationsystem 100 in which example embodiments can be employed. WCD 102 maycommunicate over an air interface 103 with a base transceiver station(BTS) 104, which is, in turn, coupled to or integrated with a basestation controller (BSC) 106. Transmissions over air interface 103 fromBTS 104 to WCD 102 may represent a “forward link” to the WCD.Conversely, transmissions over air interface 103 from WCD 102 to BTS 104may represent a “reverse link” from the WCD.

BSC 106 may be connected to a mobile switching center (MSC) 108. BSC106, MSC 108, or both, may act to control assignment of air interfacetraffic channels to WCDs, and may provide access to wirelesscircuit-switched services such as circuit-voice and circuit-dataservices. As represented by its connection to publically-switchedtelephone network (PSTN) 112, MSC 108 may also be coupled with one ormore other MSCs or other telephony circuit switches, thereby supportinguser mobility across MSC regions, as well as local and long-distancelandline telephone services. A home location register (HLR) 110, whichmay be connected to MSC 108, may support mobility-related aspects ofsubscriber services, including dynamic tracking of subscriberregistration location and verification of service privileges.

As shown, BSC 106 may also be connected with a packet-data serving node(PDSN) 116 by way of a packet control function (PCF) 114. PDSN 116, inturn, provides connectivity with a packet-switched network 118, such asthe Internet and/or a wireless carrier's private core packet-network.Nodes on network 118 may include, by way of example, an authentication,authorization, and accounting (AAA) server 120, a mobile-IP home agent(HA) 122, and a remote computer 124. After acquiring a traffic channelover air interface 103, WCD 102 may transmit a request to PDSN 116 for aconnection to the packet data network. Then, following authentication ofWCD 102 by AAA server 120, WCD 102 may be assigned an IP address by thePDSN or by HA 122, and may thereafter engage in packet-datacommunications with entities such as remote computer 124.

In some deployments, the combination of elements including BTS 104, BSC106, and MSC 108 may be referred to as a RAN. However, a RAN may containmore or fewer elements. For instance, some RANs may also include HLR110, PCF 114, PDSN 116, and/or other elements not shown in FIG. 1.

In practice, a BSC may serve multiple BTSs, each of which may thenradiate to define a wireless coverage area. Each wireless coverage area,in turn, may comprise a plurality of wireless coverage areas. Thisarrangement is illustrated in FIG. 2, which shows BSC 106 in network 100coupled with BTSs 202, 204, and 206. Each BTS is shown at the core of arespective circle representing a wireless coverage area, and eachwireless coverage area is divided into three pie-shaped piecesrepresenting wireless coverage areas. With this arrangement, a WCD mayoperate in any of the wireless coverage areas and can connect, via aserving BTS, with MSC 108 for circuit-based services and/or via PCF 114and PDSN 116 for packet-based services. Note that the depiction of threewireless coverage areas for each of the BTSs in FIG. 2 is intended to befor purposes of example, and other numbers of wireless coverage areasper BTS are possible. Further, the relative positions of the BTSs andthe relative angular orientations of the wireless coverage areas arealso illustrative, and other arrangements may be used. Moreover,wireless coverage areas need not be circular, and may take on othershapes and arrangements instead.

A WCD may receive the same bearer data simultaneously from more than onewireless coverage area. To illustrate that point, FIG. 2 includes pointsX and Y. While in the vicinity of point X, a WCD may be able to receivesignals from wireless coverage areas of BTS 202 and BTS 204. Similarly,while in the vicinity of point Y, a WCD may be able to receive signalsfrom wireless coverage areas of BTS 204 and BTS 206. It should be notedthat, in practice, a WCD located at or near points X or Y may be able toreceive signals (and therefore simultaneously receive the same bearerdata) from more than two wireless coverage areas.

In general, the depictions of both FIGS. 1 and 2 are illustrative.Therefore, in a RAN, there could be more or fewer of each element thanis shown, and some elements may be omitted altogether. Additionally,other types of elements not shown may be present. Further, any of theseelements may be combined with one another, physically or logically, ordistributed across multiple physical devices. Thus, the particulararrangement shown in FIG. 1 should not be viewed as limiting withrespect to the present invention. For instance, BSC 106 may be replacedby one or more radio network controllers (RNCs), and MSC 108 may bereplaced, in whole or in part, by one or more softswitch and/or mediagateway components.

FIG. 3 is a simplified block diagram exemplifying a RAN device 300,illustrating some of the functional components that could be included ina RAN device arranged to operate in accordance with the embodimentsherein. Example RAN device 300 could be any type of device found in orassociated with a RAN, such as a BTS, a BSC, or an MSC. For purposes ofsimplicity, this specification may equate RAN device 300 to a BSC fromtime to time. Nonetheless, it should be understood that the descriptionof RAN device 300 could apply to any component used for the purposesdescribed herein.

In this example, RAN device 300 includes a processor 302, a data storage304, a network interface 306, and an input/output function 308, all ofwhich may be coupled by a system bus 310 or a similar mechanism.Processor 302 can include one or more CPUs, such as one or more generalpurpose processors and/or one or more dedicated processors (e.g.,application specific integrated circuits (ASICs) or digital signalprocessors (DSPs), etc.).

Data storage 304, in turn, may comprise volatile and/or non-volatiledata storage and can be integrated in whole or in part with processor302. Data storage 304 can hold program instructions, executable byprocessor 302, and data that may be manipulated by these instructions tocarry out the various methods, processes, or functions described herein.Alternatively, these methods, processes, or functions can be defined byhardware, firmware, and/or any combination of hardware, firmware andsoftware. By way of example, the data in data storage 304 may containprogram instructions, perhaps stored on a non-transitorycomputer-readable medium, executable by processor 302 to carry out anyof the methods, processes, or functions disclosed in this specificationor the accompanying drawings.

Network interface 306 may take the form of a wireline connection, suchas an Ethernet, Token Ring, or T-carrier connection. Network interface306 may also take the form of a wireless connection, such as IEEE 802.11(Wifi), BLUETOOTH®, or a wide-area wireless connection. However, otherforms of physical layer connections and other types of standard orproprietary communication protocols may be used over network interface306. Furthermore, network interface 306 may comprise multiple physicalinterfaces.

Input/output function 308 may facilitate user interaction with exampleRAN device 300. Input/output function 308 may comprise multiple types ofinput devices, such as a keyboard, a mouse, a touch screen, and so on.Similarly, input/output function 308 may comprise multiple types ofoutput devices, such as a monitor, printer, or one or more lightemitting diodes (LEDs). Additionally or alternatively, example RANdevice 300 may support remote access from another device, via networkinterface 306 or via another interface (not shown), such an RS-232 orUSB port.

II. CDMA Communications

For purposes of illustration, an example that uses Code DivisionMultiple Access (CDMA) communications will be described. However, itshould be understood that other examples could use other protocolsand/or functions now known or developed in the future.

In a CDMA wireless network, each wireless coverage area may employ oneor more frequency bands, typically 1.25 MHz in bandwidth each, and eachwireless coverage area may be distinguished from adjacent wirelesscoverage areas by a pseudo-random number offset (“PN offset”). Further,each wireless coverage area may concurrently communicate on multiplechannels that are distinguished from one another by different CDMA codes(i.e., different Walsh codes). When a WCD operates in a given wirelesscoverage area, communications between the WCD and the BTS of thewireless coverage area may be carried on a given frequency and may alsobe encoded (e.g., modulated) by the wireless coverage area's PN offsetand a given Walsh code.

Air interface communications in a wireless coverage area may be dividedinto forward link communications and reverse link communications. On theforward link, certain Walsh codes may be reserved for defining controlchannels, including a pilot channel, a sync channel, and one or morepaging channels, and the remainder may be allocated dynamically for useas traffic channels, i.e., to carry bearer data such as email, webbrowsing, voice, video, and so on. Similarly, on the reverse link, oneor more offsets of a CDMA code (i.e., offsets of a PN long code) may bereserved for defining control channels, such as access channels, and theremaining offsets may be allocated dynamically to WCDs for use astraffic channels.

Channel assignment to WCDs, which typically involves allocating one ormore resources of a wireless coverage area to the WCDs, may occur when anew call (e.g., a voice, video, music, and/or data session) isestablished involving the WCD, or when the WCD hands off to a differentwireless coverage area. Each of these scenarios are described below.

a. Idle Handoff and Call Establishment

Each BTS of a RAN may emit a pilot channel signal in each wirelesscoverage area the respective BTS defines. Based on these pilot channelsignals, an idle WCD (e.g., a WCD not involved in a call) may associatewith a primary wireless coverage area, and then listen to the pagingchannel of the primary wireless coverage area for incoming callindications, and other information, from the RAN. The RAN may transmitsystem parameter messages and/or neighbor list update messages to theWCD via this primary paging channel. These messages may identify PNoffsets of the pilot channels emitted by BTSs that define neighboringwireless coverage areas (e.g., wireless coverage areas defined by theRAN's BTSs or wireless coverage areas defined by nearby BTSs indifferent RANs).

An idle WCD may measure the pilot channel signal strength that itreceives from each of these neighboring wireless coverage areas. If, forsome period of time, the WCD receives pilot channel signals from aneighboring wireless coverage area at a greater strength than the WCDreceives pilot channel signals from the primary wireless coverage area,the WCD may hand off to the neighboring wireless coverage area. To doso, the WCD may stop listening to the primary wireless coverage area'spaging channel and associate with the neighboring wireless coveragearea. Accordingly, the WCD may begin listening to the neighboringwireless coverage area's paging channel, and may transmit a radioenvironment report message to the RAN, via the neighboring wirelesscoverage area's access channel, indicating the handoff. In this way, theneighboring wireless coverage area becomes the WCD's new primarywireless coverage area.

When the WCD seeks to engage in a voice or data call, the WCD may usethe primary wireless coverage area's paging channel and access channelto set up the call. For example, when an idle WCD originates a newoutgoing call (i.e., the WCD is the caller), the WCD may transmit one ormore origination, or probe, messages to the RAN via the access channelof the primary wireless coverage area. The RAN may respond by assigninga forward-direction traffic channel to the WCD, and transmitting, viathe paging channel, an indication of this assignment (e.g., a Walsh codeof the assigned traffic channel). This transmission may take the form ofa channel assignment message directed to the WCD. Then, the WCD may usethe assigned traffic channel for receiving bearer traffic for the voiceor data call.

On the other hand, when an idle WCD is the recipient of a new incomingcall (i.e., the WCD is the callee), the RAN may transmit a page requestmessage to the WCD on the paging channel of the primary wirelesscoverage area. In response to receiving the page request message, theWCD may transmit, to the RAN, a page response message via the primarywireless coverage area's access channel. Similar to how outgoing callsare set up, the RAN may respond by assigning a forward-direction trafficchannel to the WCD, and transmitting, via the paging channel, anindication of this assignment (e.g., a Walsh code of the assignedtraffic channel) to the WCD in a channel assignment message. Then, theWCD may use the assigned traffic channel for receiving bearer traffic.

The WCD may transmit bearer data to the RAN on a reverse traffic channelby applying the WCD's assigned PN long code offset to a PN long code.The WCD may then modulate the bearer data according to the resultingpoint in the PN long code.

b. Soft Handoff

During a call, a WCD may communicate substantially simultaneously via anumber of “active” wireless coverage areas at a time. Herein, the term“substantially simultaneously” may be used to describe communicationsinvolving two or more discrete messages that pass through two or moredifferent intermediate points at approximately the same time, such aswithin a few tens of milliseconds or less.

Depending on the type and/or configuration of the RAN, the number ofactive wireless coverage areas may be from one to six. However, morethan six active wireless coverage areas may be used without departingfrom the scope of this invention. The WCD may maintain a list of theactive wireless coverage areas, identified according to their PNoffsets. This list may be referred to as the WCD's “active set.”

A RAN may be arranged to transmit the same bearer data to a given WCDconcurrently via some or all of the wireless coverage areas in the givenWCD's active set, encoding each transmission according to the PN offsetof the respective wireless coverage area and the Walsh code for theassigned channel therein. Correspondingly, the WCD may decode forwardlink transmissions from each wireless coverage area using the respectivewireless coverage area's PN offset together with the WCD's respectivelyallocated Walsh code for the wireless coverage area. The concurrenttransmissions in wireless coverage areas of the active set provides anadded level of reliability to communications, as well as possiblyincreased quality owing to improved signal-to-noise characteristics. Theconcurrency also facilitates a form of seamless handoff between wirelesscoverage areas, referred to as soft “handoff” when the handoff isbetween wireless coverage areas of different BTSs, and softer “handoff”when the handoff is between wireless coverage areas of the same BTS.(For sake of simplicity, only the term “soft handoff” will be used inthe following discussion.)

In addition to its active set, the WCD may maintain a list of“candidate” wireless coverage areas (typically up to six, but more ispossible), which includes wireless coverage areas that are not in theWCD's active set but that have sufficient signal strength such that theWCD could demodulate signals from those wireless coverage areas.Further, the WCD may maintain a list of “neighbor” wireless coverageareas that are not in its active set or candidate set, but are in closevicinity to the WCD and deemed by the RAN to be wireless coverage areasthat the WCD should monitor for eligibility as candidate wirelesscoverage areas. Other wireless coverage areas that are not in the WCD'sactive set, candidate set, or neighbor set may be members of a“remaining” set.

The WCD may continuously, or from time to time, measure the strength ofeach pilot channel signal that it receives and may notify the RAN when areceived pilot strength is above or below designated thresholds. Moreparticularly, the BTS may provide the WCD with a handoff directionmessage (HDM), which indicates (i) the PN offsets of the wirelesscoverage areas in the WCD's active set, and (ii) the following handoffparameters that relate to pilot signal strength:

-   -   T_ADD: Threshold pilot strength for addition to the active set        (e.g., −14 decibels (dB))    -   T_COMP: Difference in signal strength from an active set pilot        (e.g., 2 dB)    -   T_DROP: Threshold pilot strength for removal from the active set        (e.g., −16 dB)    -   T_TDROP: Time for which an active set pilot falls below T_DROP        to justify removal from the active set (e.g., 2 seconds)

The WCD may then monitor the pilot signals that it receives, measuringsignal strength for each as E_(c)/I_(o), where E_(c) is energy per CDMAchip for the pilot signal of a given wireless coverage area and I_(o) isthe total power received. Values of E_(c)/I_(o) may range from 0 dB(very good signal strength) to −16 dB (very poor signal strength). Itshould be understood that E_(c)/I_(o) measures a signal-to-noise ratio,but other methods of measuring signal strength, as well as other rangesof signal strength values, may be used.

If the pilot signal strength of any neighbor wireless coverage areaexceeds T_ADD, the WCD may add the pilot to its “candidate” set, andtransmit a pilot strength measurement message (PSMM) to the BSC withinformation indicative of the identity of the wireless coverage area. Ifthe pilot strength exceeds any active wireless coverage area signal byT_COMP, the BSC may then transmit an HDM to the WCD, listing the pilotas a new member of the active set. Upon receipt of the HDM, the WCD mayadd the pilot to its active set as instructed, and transmit a HandoffCompletion Message (HCM) to the BSC, acknowledging the instruction, andproviding a list of the pilots (i.e., PN offsets) in its active set.This process of the WCD adding a new wireless coverage area to itsactive set is a soft handoff.

If the WCD detects that the signal strength of a pilot channel in itsactive set drops below T_DROP, the WCD starts a handoff drop timer. IfT_TDROP passes without this signal strength exceeding T_DROP, the WCDmay then transmit a PSMM to the BSC, indicating the wireless coveragearea and the detected E_(c)/I_(o). The BSC may then respond bytransmitting an HDM to the WCD, without the wireless coverage area inthe active set. The WCD may then receive the HDM and responsively movethe wireless coverage area to its neighbor set and transmit an HCM tothe BSC.

In this way, while the WCD is actively communicating (e.g., transmittingand/or receiving bearer data), the WCD may be receiving suchcommunications from more than one wireless coverage area. Further, asthe WCD moves about or the wireless channel conditions between the WCDand its serving BTS(s) change, membership in the WCD's active set,candidate set, neighbor set, and remaining set may also change.Generally speaking, the larger the WCD's active set, the more likely itis that the WCD will receive bearer data correctly.

c. Channel Assignment

For purposes of illustration, FIG. 4 shows an example message flow 400of a RAN transmitting channel assignment messages via multiple wirelesscoverage areas during call establishment. FIG. 4 involves WCD 101, BTS202, BTS 204, and BSC 106. BTS 202 defines a wireless coverage area withPN offset 1 (“PN 1”), and BTS 204 defines another wireless coverage areawith PN offset 2 (“PN 2”). BTS 202 and BTS 204 may be controlled by BSC106. WCD 101 may be able to receive pilot channel signals of sufficientstrength from both PN 1 and PN 2 such that WCD 101 could communicateeffectively via either of these wireless coverage areas. Without loss ofgenerality, it is assumed that PN 1 is the primary wireless coveragearea of WCD 101.

WCD 101 may report, to BSC 106, measurements of the signal strengths atwhich WCD 101 receives the pilot channels of PN 1 and PN2. Thisreporting may occur through the transmission of radio environmentreports or pilot strength measurement messages (PSMMs), or via adifferent type of message.

Steps 402 and 404 illustrate BSC 106, via BTS 202, transmitting anoptional page request message to WCD 101. Such a page request messagemay be transmitted when WCD 101 is paged to answer an incoming voicecall or to receive incoming data. Steps 406 and 408 illustrate WCD 101transmitting a message via BTS 202 to BSC 106. This message may be apage response message transmitted on the access channel in response tothe optional page request message. Alternatively, this message may be anorigination message, also transmitted on the access channel, with whichWCD 101 attempts to establish an outgoing voice or data call. Additionalalternative message types may also be used for this purpose, and anysuch message may be transmitted on the access channel or another type ofchannel.

Regardless of whether an incoming or outgoing call is being establishedfor WCD 101, at step 410, BSC 106 may assign a traffic channel to WCD101. In a possible scenario, BSC 106 may assign a traffic channel fromPN 1. When making the traffic channel assignment, BSC 106 may considerpilot channel signal strength measurements that it received from WCD101. These considerations may be based on, for example, just the mostrecently-received measurement, or several recently receivedmeasurements. BSC 106 may use these received signal strengths, and/orother information, when determining from which wireless coverage area toassign a traffic channel. Thus, if BSC 106 determines that WCD 101receives the pilot signal from PN 1 at a lower strength than that of PN2, BSC 106 may instead assign a traffic channel from PN 2 to WCD 101.

At steps 412 and 414, BSC 106 may transmit a first channel assignmentmessage via BTS 202 to WCD 101. The first channel assignment message mayinclude a traffic channel assignment for PN 1. In other words, the firstchannel assignment message may instruct WCD 101 to use a particularWalsh code to receive from PN 1. Similarly, at steps 416 and 418, BSC106 may transmit a second channel assignment message via BTS 204 to WCD101. The second channel assignment message may also include a trafficchannel assignment for PN 1 (thus, these two channel assignment messagesmay serve to assign the same channel). By transmitting multiple channelassignment messages to WCD 101, the likelihood that WCD 101 receives atleast one of these messages is increased. Regardless, at step 420, WCD101 may begin receiving bearer traffic via BTS 202 (using PN 1).

While message flow 400 shows only two channel assignment messages beingtransmitted to WCD 101, more or fewer channel assignment messages may betransmitted to WCD 101 without departing from the scope of theinvention. Further, throughout message flow 400, the names of thesemessages are used for purposes of convenience and messages with othernames may be used for similar purposes.

d. Substantially Simultaneous Transmission of Bearer Data

As described in Section IIb, when the RAN substantially simultaneouslycommunicates bearer data with a WCD via more than one wireless coveragearea, the RAN and WCD may be able to engage in soft handoff procedures.Soft handoff may result in fewer dropped calls and a higher overall callquality, especially if the WCD is in motion.

Channel Assignment into Soft Handoff (CASHO) has been proposed as a wayof assignment multiple traffic channels from different wireless coverageareas to a WCD during call establishment. Thus, using CASHO proceduresmay increase the reliability and quality of the initial portions of thecalls. For purposes of illustration, FIG. 5 shows an example messageflow 500 of a RAN and WCD engaging in CASHO procedures. Like FIG. 4,FIG. 5 involves WCD 101, BTS 202, BTS 204, and BSC 106.

Steps 502 and 504 illustrate BSC 106, via BTS 202, transmitting anoptional page request message to WCD 101. Steps 506 and 508 illustrateWCD 101 transmitting a page response message or origination message viaBTS 202 to BSC 106. At step 510, BSC 106 may assign multiple trafficchannels to WCD 101. In particular, BSC 106 may assign one trafficchannel from PN 1, and another traffic channel from PN 2, to WCD 101, inaccordance with CASHO procedures. When making the traffic channelassignment, BSC 106 may consider pilot channel signal strengthmeasurements that it received from WCD 101. BSC 106 may receive thesesignal strengths in radio environment reports or PSMMs, the pageresponse or origination message of steps 506 and 508, or in some othertype of message. In any case, BSC 106 may use these received signalstrengths, and/or other information, when determining whether and/or howto perform CASHO procedures.

At steps 512 and 514, BSC 106 may transmit a channel assignment messagevia BTS 202 to WCD 101. The channel assignment message may includetraffic channel assignments for both PN 1 and PN 2. In other words, thechannel assignment message may instruct WCD 101 to use a particularWalsh code with PN 1 and another Walsh code with PN 2. Accordingly, atsteps 516 and 518, WCD 101 may begin transmitting and receiving bearerdata via both BTS 202 (using PN 1) and BTS 204 (using PN 2). Thus, viaboth BTS 202 and BTS 204, WCD 101 may receive forward direction bearerdata streams from BSC 106, and may combine these streams into a singlestream of bearer data. For example, WCD 101 may add the received signalsfrom BTS 202 and BTS 204. Conversely, via both BTS 202 and BTS 204, BSC106 may receive reverse direction bearer data streams from WCD 101, andmay also combine these streams into a single stream of bearer data.

It should be understood that rather than traversing BTS 202, any of thepage request messages, page response or origination messages, and/orchannel assignment messages may instead traverse BTS 204, or both BTS202 and BTS 204. Alternatively, BSC 106 may assign WCD 101 trafficchannels from two different PNs defined by the same BTS. Further, thenames of these messages are used for purposes of convenience andmessages with other names may be used for similar purposes.Additionally, CASHO procedures may be performed such that more than twotraffic channels are assigned to a WCD during call initiation.

III. Example Media Codecs

As noted above, a media codec may encode an analog or digital stream ofinformation (e.g., voice, video, still images, music, data, and so on)for transmission and/or storage. For example, a source WCD may include avoice codec that receives a spoken utterance from a user, and encodesthis utterance according to a particular format. The source WCD may thentransmit the encoded utterance to a destination WCD. The destination WCDmay use the same (or a similar) voice codec to decode the utterance fromthe particular format so that the destination WCD can play out theresulting signal.

Media codecs may be either lossless or lossy. Lossless media codecs mayuse an encoding format that allows the encoded media to be decoded backto its original format. Thus, lossless media codecs may support highquality transmission and storage of media.

On the other hand, some media codecs are lossy. Lossy codecs aretypically used on media for which some degree of degradation isacceptable. For instance, compact disc audio can be compressed to about10-20% of its size (i.e., achieving about 80-90% compression) by usingan MP3 codec to discard the audio components that are beyond theauditory resolution ability of most individuals. Thus, to mostlisteners, music encoded in the MP3 format sounds about the same as itwould if played directly from the compact disc. Similarly, voice codecsmay take advantage of psychoacoustics to remove redundant or lessaudible components of voice signals, resulting in about 80-90%compression of the voice signal.

In general, different lossy codecs may support different extents oflossy compression (e.g., some codecs will support compression with moreloss than other codecs). Some lossy codecs may support multiple extentsof lossy compression (e.g., a particular codec may select between two ormore rates of lossy compression).

In general, there may be a roughly linear relationship between mediacodec bit rate and the media quality that the media codec produces atthat bit rate. For example, a voice codec operating at 9.6 kilobits persecond is likely to produce better quality voice than a voice codecoperating at 4.8 kilobits per second. However, as media codectechnologies advance, new media codecs may be introduced that arecapable of supporting equal or better media quality at a lower bit rate.Thus, in some cases, a voice codec that operates at 8.5 kilobits persecond may produce better voice quality than the voice codec operatingat 9.6 kilobits per second. Furthermore, some voice codecs are capableof supporting multiple different encoding rates, and perhaps evenswitching between these rates dynamically to adapt to thecharacteristics of the input signal and/or to achieve a target bit rate.

In order to further illustrate these aspects of media codecs, severaldifferent voice codecs are compared and contrasted below. Particularly,CDMA wireless networks may use one or more codecs from the EnhancedVariable Rate Codec (EVRC) family.

For instance, the EVRC-A codec operates on input speech signals sampledwith 16-bit resolution 8,000 times per second (e.g., 8,000 Hz). Theresulting 128 kilobit per second stream is divided into 20 millisecondframes, each of which is compressed to either 171 bits (8.55 kilobit persecond), 80 bits (4.0 kilobits per second), or 16 bits (0.8 kilobits persecond). EVRC-A may also be referred to as CMDA service option 3.

The EVRC-B codec also operates on input speech signals sampled with16-bit resolution 8,000 times per second, and supports the threecompressed bit rates supported by EVRC-A. However, EVRC-B also supportsa compressed frame size of 40 bits (2.0 kilobits per second).Additionally, EVRC-B supports eight operating points, each defining atarget bit rate. When configured to operate at one of these operatingpoints, EVRC-B may attempt to achieve the desired bit rate by switchingbetween two or more of the supported frame sizes. EVRC-B may also bereferred to as CMDA service option 68.

The EVRC-WB codec is a “wideband” variation of EVRC-B. Particularly,EVRC-WB operates on input speech signals sampled with 16-bit resolutionat 8,000 or 16,000 times per second. When sampling at the rate of 8,000times per second, frames encoded with EVRC-WB can be compatible withEVRC-B encodings. When sampling at 16,000 times per second, framesencoded with EVRC-WB are 171 bits (8.55 kilobit per second). However,unlike the 171 bit frames produced when sampling at 8,000 times persecond, the EVRC-WB frames include high-frequency components from the3.5 kHz to 7 kHz range. Thus, at the same bit rate, EVRC-WB may becapable of producing higher quality voice calls than EVRC-A or EVRC-B.Additionally, EVRC-WB supports two of the operating points of EVRC-B,and also supports a mode for improved encoding of non-speech signals,such as music-on-hold. EVRC-WB may also be referred to as CMDA serviceoption 70.

The EVRC-NW codec supports at least some of the encodings of both EVRC-Band EVRC-WB. Particularly, EVRC-NW supports the sampling rates and framesizes of EVRC-WB. Also, EVRC-WB supports seven of the operating pointsof EVRC-B, and also supports the mode for improved encoding ofnon-speech signals. Thus, EVRC-NW is fully compatible with EVRC-WB, andsupports more operating modes of EVRC-B than EVRC-WB. EVRC-NW may alsobe referred to as CMDA service option 73.

The media codecs described herein are only examples. Other voice ornon-voice codecs may be used instead.

Given the benefits of some media codecs (e.g., EVRC-WB and EVRC-NW) overothers (e.g., EVRC-A and EVRC-B), the RAN may preferentially assignwireless coverage areas to a WCD so that the WCD can engage insubstantially simultaneous communication with a more advanced mediacodec, if possible. These advanced codecs may be capable of supportinghigher-quality voice calls, perhaps at a lower bit rate. Also, advancedcodecs may be capable of supporting more bit rates (e.g., more operatingmodes) as well as advanced features, such as special encoding fornon-speech signals.

For example, suppose that the WCD supports EVRC-WB and EVRC-B, and thatthe RAN has eight candidate wireless coverage areas that can be assignedto the WCD to transport the WCD's communication. Suppose further thattwo of these wireless coverage areas support both EVRC-WB and EVRC-B,while the remaining six support only EVRC-B. If the RAN assigns at leastone of the six wireless coverage areas that support only EVRC-B to theWCD, the WCD may be limited to using EVRC-B for voice calls. However, ifthe RAN assigns wireless coverage areas to the WCD only from the twothat support both EVRC-WB and EVRC-B, the WCD may be able to use EVRC-WBfor voice calls.

Therefore, when deciding which wireless coverage areas to assign to aWCD, the RAN may determine one or more codecs supported by the WCD. Forexample, at step 510 of FIG. 5, BSC 106 may determine that WCD 101supports EVRC-WB and EVRC-B. BSC 106 may make this determination basedon indications of one or more service options contained in the pageresponse or origination message of step 508. However, BSC 106 maydetermine supported media codecs in other ways as well. For instance,BSC 106 may store or have access to information that indicates one ormore media codecs supported by WCD 101. Alternatively, or additionally,BSC 106 may query WCD 101, thus triggering the WCD 101 to transmit alist of one or more supported codecs to BSC 106. This query may occurperiodically, from time to time, or in response to receiving theresource requests.

Possibly also part of step 510, BSC 106 may determine that wirelesscoverage areas of BTS 202 and BTS 204 support both EVRC-WB and EVRC-B,while wireless coverage areas of BTS 206 (not shown in FIG. 5) supportonly EVRC-B. Consequently, if BSC 106 assigns channels from one or morewireless coverage areas of BTS 206 to WCD 101, WCD 101 may be limited tocommunicating using EVRC-B. However, if BSC 106 assigns channels fromone or more wireless coverage areas of BTS 202 and/or BTS 204, WCD 101may be able to communicate using EVRC-WB. Thus, in some embodiments, BSC106 may preferentially assign channels from just wireless coverage areasof BTS 202 and/or BTS 204 to WCD 101.

In general, a RAN may preferentially assign wireless coverage areachannels to WCDs so that substantially simultaneous communication withcertain media codecs can take place. For instance, a RAN maypreferentially assign wireless coverage areas to WCDs to enable thoseWCDs to use EVRC-NW over EVRC-WB, EVRC-WB over EVRC-B, and/or EVRC-Bover EVRC-A. In this way, the RAN is more likely to provide users withhigher quality voice calls, perhaps at lower bit rates and with supportfor efficient processing of non-speech signals. The RAN may apply thesepreferential assignments to new calls, to calls that are being handedoff, or both.

IV. Example Operations

FIGS. 6 and 7 are flow charts depicting example embodiments. One or moresteps of either of both of these example embodiments may be carried out,for instance, by a RAN component exemplified by RAN device 300.

At step 600 of FIG. 6, a RAN may receive a call setup request from aWCD. The call setup request may indicate that the WCD supports a firstmedia codec. Alternatively, the RAN may determine the media codec(s)supported by the WCD in some other fashion. At step 602, the RAN mayobtain a set of candidate wireless coverage areas for serving the WCD. Afirst subset of the candidate wireless coverage areas may support thefirst media codec, and a second subset of the candidate wirelesscoverage areas might not support the first media codec.

At step 604, based on (i) the WCD and the first subset of the candidatewireless coverage areas supporting the first media codec, and (ii) thesecond subset of the candidate wireless coverage areas not supportingthe first media codec, the RAN may assign traffic channels to the WCD.The assigned traffic channels may include traffic channels from at leasttwo of the first subset of the candidate wireless coverage areas, andmight not include traffic channels from any of the second subset of thecandidate wireless coverage areas.

Assigning traffic channels to the WCD may involve the RAN transmittingchannel assignment messages to the WCD via at least two of the candidatewireless coverage areas. When doing so, the RAN might not transmitchannel assignment messages to the WCD via any of the second subset ofthe candidate wireless coverage areas.

In some embodiments, the second subset of the candidate wirelesscoverage areas may support a second media codec, and the first mediacodec may be capable of producing higher voice quality voice calls thanthe second media codec. For instance, the first media codec may use afirst sampling rate of over 8,000 Hz, and the second media codec may usea second sampling rate of 8,000 Hz or less. The RAN may assign theassigned traffic channels to the WCD also based on the first media codecbeing capable of producing higher voice quality voice calls than thesecond media codec.

Further, the first media codec may be capable of producing higher voicequality voice calls at a lower bit-rate than the second media codec. TheRAN may assign the assigned traffic channels to the WCD also based onthe first media codec being capable of producing higher voice qualityvoice calls at a lower bit-rate than the second media codec.

Alternatively or additionally, the first media codec may be capable ofsupporting a larger number and/or range of bit rates than the secondmedia codec. The RAN may assign the assigned traffic channels to the WCDalso based on the first media codec being capable of supporting a largernumber and/or range of bit rates than the second media codec.

At step 606, the RAN may communicate with the WCD substantiallysimultaneously via the assigned traffic channels. The communicationbetween the RAN and the WCD may use the first media codec. The RANcommunicating with the WCD substantially simultaneously via the assignedtraffic channels may involve, during an initial portion of thecommunication between the RAN and the WCD (e.g., the first fewmilliseconds or the first few seconds of a call), the RAN exchangingbearer traffic with the WCD via each of the assigned traffic channels.For instance, the RAN may transmit and receive the same bearer trafficvia each of the assigned traffic channels. Alternatively oradditionally, the RAN communicating with the WCD substantiallysimultaneously via the assigned traffic channels may involve thesubstantially simultaneous communications traversing at least two of thefirst subset of the candidate wireless coverage areas at approximatelythe same time.

FIG. 7 depicts another example embodiment. At step 700, a RAN mayreceive a handoff request from a WCD. The handoff request may indicatethat the WCD supports a first media codec. The WCD may be served by afirst set of wireless coverage areas, each of which supports the firstmedia codec.

At step 702, a second set of wireless coverage areas that are candidatesfor adding to the first set may be obtained. A particular candidatewireless coverage area may support the first media codec, and at leastone other candidate wireless coverage area might not support the firstmedia codec.

At step 704, based on (i) the WCD and the particular candidate wirelesscoverage area supporting the first media codec, and (ii) the at leastone other candidate wireless coverage area not supporting the firstmedia codec, the RAN may add the particular candidate wireless coveragearea to the first set. Additionally, the RAN might not add the at leastone other candidate wireless coverage area to the first set.

At step 706, after adding the particular candidate wireless coveragearea to the first set, the RAN may communicate with the WCDsubstantially simultaneously via the first set of wireless coverageareas. The communication between the RAN and the WCD may use the firstmedia codec.

It should be understood that FIGS. 6 and 7 depict non-limitingembodiments. Thus, more or fewer steps than shown in FIGS. 6 and 7 maybe used without departing from the scope of the embodiments herein.Additionally, some of these steps may be repeated one or more times, ormay be omitted altogether. Further, the message flows and flow charts ofthe figures may be combined with one another and/or with other aspectsdescribed this specification and its accompanying drawings, in whole orin part, also without departing from the scope of the embodimentsherein. For instance, any of the features discussed in the context ofFIG. 6 may also be applied to methods illustrated by the flow chart ofFIG. 7.

In the drawings, a step or block that represents a processing ofinformation may correspond to circuitry that can be configured toperform the specific logical functions of a herein-described method ortechnique. Alternatively or additionally, a step or block thatrepresents a processing of information may correspond to a module, asegment, or a portion of program code (including related data). Theprogram code may include one or more instructions executable by one ormore processors for implementing specific logical functions or actionsin the method or technique. The program code and/or related data may bestored on any type of computer-readable medium, such as a storagedevice, including a disk drive, a hard drive, or other storage media.

V. Conclusion

Example embodiments have been described above. Those skilled in the artwill understand, however, that changes and modifications may be made tothese embodiments herein without departing from the true scope andspirit of the invention, which is defined by the claims.

What is claimed is:
 1. A method comprising: a radio access network (RAN)receiving a call setup request from a wireless communication device(WCD), wherein the call setup request indicates that the WCD supports afirst media codec; the RAN obtaining a set of candidate wirelesscoverage areas for serving the WCD, wherein a first subset of thecandidate wireless coverage areas support the first media codec, and asecond subset of the candidate wireless coverage areas do not supportthe first media codec; based on (i) the WCD and the first subset of thecandidate wireless coverage areas supporting the first media codec, and(ii) the second subset of the candidate wireless coverage areas notsupporting the first media codec, the RAN assigning traffic channels tothe WCD, wherein the assigned traffic channels include traffic channelsfrom at least two of the first subset of the candidate wireless coverageareas, and wherein the assigned traffic channels do not include trafficchannels from any of the second subset of the candidate wirelesscoverage areas; and the RAN communicating with the WCD substantiallysimultaneously via the assigned traffic channels, wherein thecommunication between the RAN and the WCD uses the first media codec. 2.The method of claim 1, wherein the second subset of the candidatewireless coverage areas support a second media codec, wherein the firstmedia codec is configured to produce higher voice quality voice callsthan the second media codec, and wherein the RAN assigns the assignedtraffic channels to the WCD also based on the first media codec beingconfigured to produce higher voice quality voice calls than the secondmedia codec.
 3. The method of claim 2, wherein the first media codec isconfigured to produce higher voice quality voice calls at a lowerbit-rate than the second media codec, and wherein the RAN assigns theassigned traffic channels to the WCD also based on the first media codecbeing configured to produce higher voice quality voice calls at a lowerbit-rate than the second media codec.
 4. The method of claim 2, whereinthe first media codec uses a first sampling rate of over 8000 Hz, andwherein the second media codec uses a second sampling rate of 8000 Hz orless.
 5. The method of claim 1, wherein the first media codec isconfigured to support a larger range of bit rates than the second mediacodec, and wherein the RAN assigns the assigned traffic channels to theWCD also based on the first media codec being configured to support alarger range of bit rates than the second media codec.
 6. The method ofclaim 1, wherein the RAN assigning traffic channels to the WCD comprisesthe RAN transmitting channel assignment messages to the WCD via at leasttwo of the candidate wireless coverage areas.
 7. The method of claim 6,wherein the RAN assigning traffic channels to the WCD comprises the RANtransmitting channel assignment messages to the WCD via at least two ofthe first subset of the candidate wireless coverage areas, and nottransmitting channel assignment messages to the WCD via any of thesecond subset of the candidate wireless coverage areas.
 8. The method ofclaim 1, wherein the RAN communicating with the WCD substantiallysimultaneously via the assigned traffic channels comprises during aninitial portion of the communication between the RAN and the WCD, theRAN exchanging bearer traffic with the WCD via each of the assignedtraffic channels.
 9. The method of claim 8, wherein the RAN transmitsand receives the same bearer traffic via each of the assigned trafficchannels.
 10. The method of claim 1, wherein the RAN communicating withthe WCD substantially simultaneously via the assigned traffic channelscomprises the substantially simultaneous communications traversing eachat least two of the first subset of the candidate wireless coverageareas at approximately the same time.
 11. An article of manufactureincluding a non-transitory computer-readable medium, having storedthereon program instructions that, upon execution by a radio accessnetwork (RAN) device, cause the RAN device to perform operationscomprising: receiving a call setup request from a wireless communicationdevice (WCD), wherein the call setup request indicates that the WCDsupports a first media codec; obtaining a set of candidate wirelesscoverage areas for serving the WCD, wherein a first subset of thecandidate wireless coverage areas support the first media codec, and asecond subset of the candidate wireless coverage areas do not supportthe first media codec; based on (i) the WCD and the first subset of thecandidate wireless coverage areas supporting the first media codec, and(ii) the second subset of the candidate wireless coverage areas notsupporting the first media codec, assigning traffic channels to the WCD,wherein the assigned traffic channels include traffic channels from atleast two of the first subset of the candidate wireless coverage areas,and wherein the assigned traffic channels do not include trafficchannels from any of the second subset of the candidate wirelesscoverage areas; and communicating with the WCD substantiallysimultaneously via the assigned traffic channels, wherein thecommunication between the RAN and the WCD uses the first media codec.12. The article of manufacture of claim 11, wherein the second subset ofthe candidate wireless coverage areas support a second media codec,wherein the first media codec is configured to produce higher voicequality voice calls than the second media codec, and wherein the RANassigns the assigned traffic channels to the WCD also based on the firstmedia codec being configured to produce higher voice quality voice callsthan the second media codec.
 13. The article of manufacture of claim 12,wherein the first media codec is configured to produce higher voicequality voice calls at a lower bit-rate than the second media codec, andwherein the RAN assigns the assigned traffic channels to the WCD alsobased on the first media codec being configured to produce higher voicequality voice calls at a lower bit-rate than the second media codec. 14.The article of manufacture of claim 12, wherein the first media codecuses a first sampling rate of over 8000 Hz, and wherein the second mediacodec uses a second sampling rate of 8000 Hz or less.
 15. The article ofmanufacture of claim 11, wherein the first media codec is configured tosupport a larger range of bit rates than the second media codec, andwherein the RAN assigns the assigned traffic channels to the WCD alsobased on the first media codec being configured to support a largerrange of bit rates than the second media codec.
 16. The article ofmanufacture of claim 11, wherein assigning traffic channels to the WCDcomprises the transmitting channel assignment messages to the WCD via atleast two of the candidate wireless coverage areas.
 17. The article ofmanufacture of claim 16, wherein assigning traffic channels to the WCDcomprises transmitting channel assignment messages to the WCD via atleast two of the first subset of the candidate wireless coverage areas,and not transmitting channel assignment messages to the WCD via any ofthe second subset of the candidate wireless coverage areas.
 18. Thearticle of manufacture of claim 11, wherein communicating with the WCDsubstantially simultaneously via the assigned traffic channels comprisesduring an initial portion of the communication between the RAN and theWCD, the RAN exchanging bearer traffic with the WCD via each of theassigned traffic channels.
 19. The article of manufacture of claim 18,wherein the RAN transmits and receives the same bearer traffic via eachof the assigned traffic channels.
 20. The article of manufacture ofclaim 11, wherein the RAN communicating with the WCD substantiallysimultaneously via the assigned traffic channels comprises thesubstantially simultaneous communications traversing each at least twoof the first subset of the candidate wireless coverage areas atapproximately the same time.